Collections of items can be arranged or categorized into nodes. For example, a collection of products can be arranged into a node that represents a product category or product attribute. The quality of the node can be judged, in one way, in terms of the homogeneity of the node. In other words, the quality of the node can be judged in terms of how related the items that are members of the node are to one another. In large collections of items that are associated with multiple nodes, it may be the case that some nodes are of higher quality than other nodes. In other nodes, some nodes may be more homogeneous than other nodes. Additionally, some nodes may contain outlier items, or items that are not ideally placed within a particular node.